B cell immune responses are regulated by a family of T cell and macrophage derived glycoproteins. Here we extend our understanding of this regulation by demonstrating the existence of two distinct T cell derived B cell growth factors which can be separated biochemically and which appear to operate on B cells at different stages of activation. We demonstrate that in contrary to previous beliefs suggesting only Lyb 5+ B cells are regulated by factors, in fact most if not all B cell are governed by these mediators. Our preliminary data indicate that specific induction of resting B cells via cross-linkage of membrane Ig receptors renders them apparently unresponsive to the family of non-specific maturation factors which drive resting B cells to rapid immunoglobulin synthesis. Finally, we are currently developing assays for the biochemical identification of the receptor for BCGF-I.